A known inkjet printing system is illustrated in FIGS. 1 through 4. The external mechanical construction of an inkjet print head 1 is apparent in FIG. 1. The central area of the inkjet print head forms an elongated plate 2 with adjustment and/or fastening holes 3, 4 and/or fastening pins in end areas 5 of the plate 2 on the end-face side.
Situated between these end areas 5 used for adjustment and fastening is a central, preferably thickened portion 6 of the plate 2, where a plurality of nozzles 8 for delivering ink drops in the direction toward the substrate to be imprinted open into the flat side 7 of the plate 2 facing a substrate to be imprinted. In the area of the flat side 7, the middle portion 6 of the plate 2 provided with nozzles 8 may be elevated with respect to the adjoining end areas 5, so that heads 9 of fastening screws protruding beyond the end areas 5 do not contact the substrate to be imprinted.
A mechanism, illustrated in cross section in FIG. 2, extends in the area of the middle portion 6 of the plate 2 on the oppositely situated flat side 10 of the plate 2. It is apparent that a printing system 11 associated with the nozzle 8 is situated behind each nozzle.
Each printing system 11 has its own ink chamber 12, from which only the single nozzle 8 that opens into that location is supplied with ink. This ink chamber 12 is connected to a feeding ink reservoir 14 via an ink channel 13 having a comparatively much smaller cross section; the ink chamber 12 may be refilled through the ink channel 13 after ejecting an ink drop.
Whereas the ink channel 13 opens into the ink chamber 12 from the side, a diaphragm 15 which is fixed, for example clamped, only along its peripheral edge is situated at the side of the ink chamber 12 opposite from the nozzle 8. The movable portion of a piezo element 16 is fastened to the side of the diaphragm 15, in the middle area thereof, facing away from the ink chamber 12, and the piezo element in turn is fixed to a solid rear plate or to a solid rear block 17.
Via a control circuit 18 that is directly connected to the piezo element 16, the piezo element may be excited to undergo contractions or expansions, which are transmitted 1:1 to the connected diaphragm 15 and which therefore increase or decrease the volume within the ink chamber 12.
When the ink chamber 12 becomes larger, ink is drawn from the ink reservoir 14 and into the ink chamber 12; when the ink chamber 12 becomes smaller, an ink drop is ejected from the nozzle 8 when the reduction in volume is sufficiently strong and powerful, i.e., sufficiently great and rapid, so that an ink drop consequently bulging from the nozzle 8 falls off.
FIG. 3 shows that overall, only a few ink reservoirs 14 are provided, preferably only one or two, to which a larger number of ink chambers 12 are in each case connected.
It is apparent in FIG. 4 that the piezo elements 16, from an electrical standpoint, may represent capacitors that charge when connected to a supply voltage 19, in order to cause a mechanical reaction, for example a contraction or expansion; they may also be discharged or recharged to trigger the reverse mechanical reaction, for example by short-circuiting the two electrical terminals of a capacitor or by active application of a different voltage.
The totality of a printing system 11, i.e., its ink chamber 12, diaphragm 15, piezo element 16, and the control circuit 18 form an oscillatable system. The natural frequency fres off this system depends on the geometric design of the printing system 11 and the properties of its components 12, 15, 16, 18. However, the natural frequency fres does not have to be calculated; instead, after triggering an oscillation by means of a sufficiently high-energy actuating signal, the natural frequency may be read off at the electrical terminals of the piezo element 16. At this location a largely undamped oscillation is discernible, whose period Tres is inversely proportional to the natural frequency fres: fres=1/Tres.
A measuring circuit for determining this natural frequency fres is quite simple: An active control circuit 18 for a piezo element generates an isolated trigger pulse or a series of trigger pulses with widely spaced time intervals, for example at an interval of one or several seconds. If the signal pattern is so strongly attenuated after emission of the pulse, or in each case a pulse, from the control circuit 18 that the natural frequency oscillation is already dying down within an oscillation period, the connection between the control circuit 18 and the piezo element 16 may be additionally interrupted after a pulse is emitted, or interrupted until the next pulse, so that the piezo element 16 together with the connected mechanical components is in the meantime left on its own and may freely oscillate without appreciable attenuation, so that a plurality of measurable oscillation waves follow one another in succession. In this phase, an electrical voltage in the form of a gradually dying down wave is measurable at the electrical terminals of the piezo element 16, with a frequency that corresponds to the resonance frequency fres of the overall system made up of electrical and mechanical components.
The voltage at the piezo element is dynamically measured or recorded, for example with an oscillograph or a storage oscilloscope, the image display or recording being triggered by the trigger pulse. The period Tres of a resonance oscillation may then be read off on the time scale of the oscillograph screen or a stored signal recording, and the resonance frequency fres may be determined therefrom using the formula: fres=1/Tres.
For variable darkening of a pixel, up to k ink drops may be delivered per pixel:n=0,1,2, . . . k. 
The time interval Tdrop between two successive ink drops may be constant, for example Tseq/(k−1), i.e., a portion of the overall image drop sequence Tseq or a multiple thereof, in particular:Tdrop=Tseq*(k−n)/(k−1),when n<k ink drops are printed per pixel sequence.
Typically, for each pixel sequence, initially the natural oscillation is excited at the resonance frequency res off the printing system 11 by a signal having a reduced amplitude, and in the period split specified thereby, by means of an appropriately set actuation one ink drop per period Tres of the natural frequency is then delivered, in particular in each case approximately in the same phase of the natural oscillation. This entails an actuation in such a way that, in any case for more than one ink drop per pixel sequence, the following applies:Tdrop=Tres.
The printing system is thus subordinate to the natural frequency of the system; this natural frequency, in a manner of speaking, forms the clock pulse in which printing takes place. However, it has been shown that this pulse frequency fres is comparatively comparatively slow, and therefore limits the printing speed. As a result, in turn the number of ink drops per image sequence in practice is reduced to the greatest extent possible in order to keep the printing speed within acceptable limits; however, this results in reduced printing accuracy, since each ink drop must then have a comparatively large volume, and therefore fine gradations are not possible.